A theoretical study of the H2O and O-2 adsorption on an illuminated TiO2 anatase surface is presented. The electronic structure and the spin distribution were examined by employing the DFT formalism and the BHandHLYP functional. The adsorbates geometries were fully optimized, including the cluster relaxation. Our results show the dissociative adsorption of the H2O Molecule on the photoactivated TiO2 (0 0 1) surface. This reaction produces one hydroxyl group with radical character (OH circle) and an unpaired electron localized in the 5c-Ti atom. In case of the 02 Molecule, the non-dissociative chemisorption was obtained. This molecule shows one unpaired electron and a negative charge. In these sense, the adsorbed O-2 acts as a superoxide species (O-2(circle-)). (C) 2007 Elsevier B.V. All rights reserved.